JP2009521786A - LED-based multicolor polarized illumination light source - Google Patents

Abstract

A light emitting diode (LED) emits light of a first wavelength. The phosphor material proximate to the LED converts at least a portion of the light to the second wavelength. Both wavelengths of light pass through the collection unit. The reflective polarizer transmits both wavelengths in the first polarization state and reflects light of both wavelengths in the second polarization state orthogonal thereto. Light reflected by the reflective polarizer is directed back toward the phosphor material without substantially increasing the angular range. The resulting outgoing light beam includes polarized light of a first wavelength and a second wavelength. In some embodiments, the array of LEDs includes LEDs that emit light of a first wavelength and other LEDs that emit light of a second wavelength. A phosphor material is disposed on the LED that emits light of the first wavelength.

Description

  The present invention relates to an illumination system that can be used in an image projection system. More particularly, the present invention relates to an illumination system that includes an array of light emitting elements such as light emitting diodes (LEDs) for generating polarized light.

  Illumination systems may be found in many different applications, including image projection display systems, backlights for liquid crystal displays, and the like. Typically, a projection system includes a light source, illumination optics that allow light to pass through one or more image forming devices, projection optics for projecting an image from the image forming device, and a projection screen on which the image is displayed. use. The image forming apparatus is controlled by a video signal that is electrically adjusted and processed.

  White light sources, such as high pressure mercury lamps, are still the main light sources used in projection display systems. In a three-plate image projection system, a white light beam is split into three primary color channels of red, green, and blue and directed to a respective image forming device panel that produces an image for each color. The resulting primary color image beam is combined with the full color image beam projected onto the display. Some other projection systems use a single image panel and use a rotating color wheel or some number of filters to filter white light so that one primary color light is incident on the imaging device at any point in time. Other types of time series color filters are used. The light incident on the panel changes its color sequentially and forms a colored image in synchronization with the incident light. The observer's eyes integrate the sequentially colored images and recognize them as full-color images.

  More recently, light emitting diodes (LEDs) have been considered as an alternative to white light sources. For example, different illumination channels are propelled by engines with each colored LED or LED array. For example, a blue LED is used for the blue channel, and a red LED is used for the red channel. Some image display devices, such as liquid crystal displays (LCDs), use polarized light, while LEDs produce unpolarized light, so only half of the generated light is utilized in the LCD. In addition, LEDs operating in the green region of the visible spectrum are known to be relatively inefficient compared to blue and red LEDs, where many systems require green LEDs rather than blue or red LEDs. This problem of inefficiency in the green part of the spectrum is exacerbated when the light is required to be polarized.

  One embodiment of the present invention is directed to an illumination light source that includes an array of one or more LEDs including light emitting diodes (LEDs) capable of generating light at a first wavelength. A phosphor material is disposed adjacent to the one or more LEDs. The phosphor material emits light of the second wavelength when illuminated with light of the first wavelength. The light collecting unit includes at least one tapered optical element. The light of the first and second wavelengths passes through the light collecting unit. A reflective polarizer is disposed to transmit light of the first and second wavelengths in the first polarization state and reflect light of the first and second wavelengths in the second polarization state orthogonal to the first polarization state. The first and second wavelengths of light reflected by the reflective polarizer are directed back toward the phosphor material without substantially increasing the angular range. The illumination light source generates an outgoing light beam including polarized light having first and second wavelengths.

  Another embodiment of the invention includes one or more light emitting diodes including a first LED capable of emitting light at a first wavelength and a second LED capable of emitting light at a second wavelength different from the first wavelength. An illumination light source comprising an array of (LEDs) is intended. As a result, the phosphor material is disposed on the first LED such that light from the first LED is incident on the phosphor material. The light of the first wavelength is converted to the third wavelength by the phosphor.

The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The following drawings and best mode for carrying out the invention further specifically illustrate these embodiments.

  The present invention can be used for an illumination system, and is particularly applicable to an illumination system for displaying an image, such as a projection system that can be used for a projection television and a display, a monitor, and the like.

  It is well known that green light emitting diodes (LEDs) are less efficient than LEDs operating in the blue and red regions of the visible spectrum. Thus, LED based lighting systems require more green LEDs than blue and red LEDs to achieve the desired level of brightness and color balance. Instead of generating green directly in the LED, there is another approach to generate light at a first wavelength, eg, blue or UV wavelength, and converting the first wavelength light to green wavelength light.

  One exemplary approach that may be useful for wavelength converting light from an LED to generate green light is schematically illustrated in FIG. The exemplary lighting system 100 has an array of one or more LEDs 102 mounted on a base plate 104. Base plate 104 may be used to provide power to LED 102 and to remove heat from the LED.

  At least a part of the light 106 from the LED 102 is collected in the first light collecting unit 107. In the exemplary embodiment, light collection unit 107 includes a light pipe 108 having an entrance side 110 and an exit side 112.

  A side view of the array of LEDs 102 mounted on the base plate 104 is schematically illustrated in FIG. Some commercially available LEDs that may be used in the lighting system emit light through the top surface facing the light pipe 108. Other types of commercially available LEDs emit light from the oblique surface 202 of the LED chip as shown.

  In some exemplary embodiments, the reflective element 114 may be positioned proximate to the LED array. The reflective element 114 surrounds at least a portion of the incident side 110 and reduces the amount of light that leaks from the incident side 110 of the light pipe 108. It may be desirable for the reflective element 114 to be separated from the LED 102 at a slight distance due to interference of the connection 204, for example, where the incident side 110 is used to make an electrical connection with the top of the LED 102. This configuration with the reflective element 114 allows a reduced number of LEDs 102 to be used, thus reducing cost and power consumption even when the light pipe 108 is replenished. The reflective element 114 may include a metallized or multilayered reflective coating.

  In some exemplary embodiments, the light pipe 108 is a tapered solid right angle prism placed directly above the LED 102. The incident side 110 of the light pipe 108 may be reduced to avoid increasing the etendue of the system. The etendue is the product of the area of the light beam at the light source multiplied by the solid angle of the light beam. The light etendue cannot be reduced, but can be increased by the optical system. This reduces the overall brightness of the light that illuminates the display, as the brightness is given by the optical flux divided by etendue. Thus, for example, if the area of the light beam is increased to reinforce the active area of the imager device, it is sufficient to reduce the angular range of the beam proportionally to keep the light beam etendue constant. It is. By keeping the etendue constant, the brightness of the illumination light incident on the imager device is maintained at or near the highest level achievable.

  In the exemplary embodiment shown in FIG. 1, the highest flux per etendue is such that the LED 102 is an unencapsulated LED chip and the LED 102 is separated from the incident side 110 of the light pipe 108 by an air gap. Obtained when 106 is radiated into the air in the absence of additional epoxy, silicone or other intervening material. This configuration can improve the reliability of the LED 102 by removing organic and polymer layers that can be degraded by high temperatures and light flux. In some embodiments, it may be desirable to include several capsule formations between the LED 102 and the incident side 110, for example for environmental conservation.

  It has been found that light collection efficiency in the range of 80% to 90% can be obtained by the light pipe 108 in collecting light emitted by the LED 102. In some exemplary embodiments, the light pipe 108 may have a length that is 2 to 10 times longer than its width on the exit side, but the light pipe 108 may operate outside this range. As the length of the light pipe 108 increases, the light uniformity on the exit side 112 increases. However, if the light pipe 108 becomes too long, the system becomes bulky and expensive, and less light exits the light pipe 108 due to losses inside the light pipe 108. For example, other configurations of the light pipe 108 such as a hollow tunnel can be used instead of the solid light pipe.

  In some embodiments, the collection unit 107 may also include a focusing optic 116 such as a lens on the exit side 112. The focusing optic 116 may be separated from the light pipe 108 or integrated with the light pipe 108.

  The light 118 emitted from the light collecting unit 107 may be substantially telecentric. The term “telecentric” means that the angular range of light is substantially the same for different points across the beam. Thus, if a part of the beam on one side of the beam contains light that is in a light cone with a special angular range, the other parts of the beam, eg the middle part of the beam and the other side of the beam are substantially Includes light in the same angular range. Therefore, the light at the center of the light beam is mainly directed along the axis and contained within a certain cone angle, while at the edge of the beam the light is also directed along the axis and substantially the same. The light beam is telecentric. If the light pipe 108 is long enough, the light 118 on the exit side 112 can be sufficiently telecentric without the need for focusing optics 116. Use of the focusing optical unit 116 allows the condensing unit 107 to be shorter while further generating telecentric emission. If the light is telecentric, the fraction of light that is subsequently concentrated in the phosphor for frequency conversion is increased.

  In some exemplary embodiments, the focusing optics 116 may be integrated with the light pipe 108 or separated from the light pipe 108. In other exemplary embodiments, the light pipe 108 may be equipped with curved sidewalls that perform a focusing function.

  Light 118 enters a polarizing beamsplitter (PBS) 120. The PBS may be any suitable type of PBS, for example, MacNeille-type PBS or US Pat. Nos. 5,962,114 and 6,697, incorporated herein by reference. It may also be a multilayer optical film (MOF) such as MZIP PBS described in 721,096. Other suitable types of PBS include wire grid type PBS and cholesteric type PBS. PBS 120 typically includes a polarization selective layer 122 disposed between the hypotenuse surfaces of two right angle prisms 124a and 124b, although other configurations may be used. The polarization selection layer 122 reflects light in one polarization state and transmits light in a polarization state orthogonal to the light. The PBS 120 may also include a reflective film 123 disposed between the polarization selection layer 122 and the second prism 124b. The reflective layer 123 reflects the light from the LED 102 that has passed through the polarization selection layer 122. The reflective film 123 reflects light of the first wavelength generated by the LED 102 and transmits light of the second wavelength generated by the phosphor: the configuration of this reflector is sometimes referred to as a long pass reflective filter.

  As will become apparent below, in this particular embodiment, PBS 120 is used to polarize the second wavelength of light generated by the phosphor, and the action of PBS on the first wavelength of light 118 is essential. You can ignore it. For example, in some embodiments, the polarization selective layer 122 may be designed to be essentially transparent to both polarization states of the first wavelength of light 118. In such a case, the reflective film 123 reflects both polarization states of the light 118 having the first wavelength. In another embodiment, the polarization selection layer 122 reflects light of a first wavelength in one polarization state, in which case the reflective film 123 is in a second polarization state that is transmitted through the polarization selection layer 122. Reflects light 118 of wavelength.

The first wavelength light 126 reflected by the PBS 120 is directed to the color conversion phosphor 128. The phosphor 128 includes a material that absorbs the light 126 generated by the LED 102 and generates light of a second wavelength, typically longer than the first wavelength. In some embodiments, the phosphor 128 may convert blue or UV light to green light. One particularly suitable example of a phosphor material is Eu-doped strontium thiogallate (SrGa ₂ S ₄ : Eu), although other types of phosphor materials may be used, for example, rare earth elements There are oxynitrides such as doped nitride and europium doped silicon aluminum oxynitride (SiAlON: Eu), and rare earth doped garnets such as cerium doped yttrium aluminum garnet (Ge: YAG).

  The light 126 may pass through the second light collector 130 on the way to the phosphor 128. The second light concentrator 130 may have a configuration similar to the first light concentrator 107 having the focusing optical component 132 and the light pipe 134, or may have a different configuration. Focusing optics 132 and light pipe 134 concentrate light 126 on phosphor 128.

  The phosphor 128 may in some embodiments be mounted on a base plate 136 that acts as a heat sink to remove excess heat. The second wavelength light 138 (broken line) is directed to the PBS 120 via the second light collector 130, and the PBS 120 transmits the p-polarized light 140 as the effective output 142 and reflects the s-polarized light 144. Several elements behind the phosphor 128 can be used to reflect the second wavelength of light that is generated while initially moving away from the PBS 120. For example, the base plate 136 may itself be reflective, or an optional reflector 152 may be placed between the phosphor 128 and the base plate 136. One example of a suitable reflector 152 includes a metal coating on the base plate 136, such as a silver coating. Another example of reflector 152 is an enhanced specular reflector film (ESR) film available from 3M Company of St. Paul, Minnesota.

  The reflection filter 146 that transmits the first wavelength and reflects the second wavelength is disposed between the PBS 120 and the first light collector 107, and can reflect the s-polarized light 142 and return it to the phosphor 128 via the PBS 120. . The reflected light 150 may then be re-reflected by, for example, the phosphor 128, the base plate 136, or the reflector 152 and back toward the PBS.

  A polarization converter 148 is disposed between the PBS 120 and the phosphor 128 such that at least a portion of the light 150 reflected back to the phosphor 128 is subsequently returned to the PBS in a polarization state that is transmitted as an effective output 142. Can do.

  One feature of the system 100 illustrated in FIG. 1 is that the first wavelength of light 126 is incident on a phosphor having an etendue substantially similar to the light 106 emitted by the LED 102. Thus, the etendue of the second wavelength outgoing light 142 is similar to that which would be obtained with the second wavelength light generated directly using a suitable LED. This allows the emitted light 142 to be efficiently used for illumination applications, for example, illuminating an LCD imager device.

  Another exemplary embodiment of an illumination system 300 comprising a phosphor for wavelength conversion of light and including generating polarized emission is schematically illustrated in FIG. In this embodiment, light from the LED 102 passes through a collection / focusing unit 307 that includes a light pipe 308 having an entrance side 310 and an exit side 312. The light pipe 308 in this particular embodiment has curved sidewalls so that the light 314 at the exit side 312 is substantially telecentric. The light 314 passes through the dichroic beam splitter 320 having the characteristics of reflecting the first wavelength light and transmitting the second wavelength light. The first wavelength light 324 reflected by the dichroic beam splitter is directed to the phosphor 128 through the second light collecting unit 326. The second light collecting unit 326 may also include a light pipe 328 having curved side walls, or may have a different optical arrangement than the light pipe of the first light collecting unit 307.

  The light 329 having the second wavelength passes through the second light collecting device 326 and passes through the dichroic beam splitter 320. For example, a polarizer 330 such as a wire grid polarizer, a MOF polarizer, or a cholesteric polarizer transmits light 332 in one polarization state as an effective output, and transmits light 334 in a polarization state orthogonal to the light 334 in the direction of the phosphor 128. Reflect back to. A polarization control element 336, for example a quarter wave plate, may be disposed between the polarizer 330 and the dichroic beam splitter 320. The reflected light 334 again enters the phosphor 128 and is reflected back toward the polarizer 330 by the phosphor 128, the base plate 136, or the reflector 152. A polarization control element 336 is used to rotate at least some polarization of the light that is recycled to the polarizer 330.

  Further, at least a portion of the first wavelength light that is not converted to the second wavelength by the phosphor 128 is reflected by one of the phosphor 128, the reflector 152, or the base plate 136, and then reflected by the dichroic beam splitter 320. In some cases, the LED 102 may be returned via. Such reflected light of the first wavelength can be recycled to the phosphor 128 by reflection from the base plate 104 or the LED 102.

  In another exemplary embodiment, although not illustrated, the dichroic beam splitter may transmit light of the first wavelength and reflect light of the second wavelength. In such a configuration, the LED and phosphor are typically placed on the opposite side of the dichroic beam splitter.

  In some exemplary embodiments, the phosphor may be placed in close proximity to the LED, or the LED may be evenly coated with a phosphor material. Such a configuration may result in a reduction in the number of elements used in the lighting system. Also, in some cases, the LED is formed of a material such as silicon carbide, which is effective in transferring heat from the phosphor to the base plate.

  One exemplary embodiment of a lighting system 400 where the phosphor is placed in close proximity to the LED is schematically illustrated in FIG. The system includes an array of one or more LEDs 102 and a light collection unit 107. The light collection unit may be configured differently from the illustrated embodiment, for example using a light pipe integrated with the focusing element or a light pipe without a focusing element. In addition, the side walls may be straight or curved. In an exemplary embodiment, the phosphor 428 is placed in proximity to the LED 102 or even on top of the LED. The reflection filter 430 may be disposed on the emission side of the light collecting unit 107, reflects the first wavelength light 106, and transmits the second wavelength light 432 generated by the phosphor 428.

  The light 432 of the second wavelength is incident on the PBS 420, and the PBS transmits the light 434 in one polarization state as an effective output, and reflects the light 436 in a polarization state orthogonal thereto. The reflector 438 reflects the light 440 and returns it to the PBS 420, where the light is reflected back toward the phosphor 428. The light 440 may then be reflected back toward the PBS 420 by the phosphor 428, the LED 102, the base plate 102, or some other reflective element. A polarization rotation element 442, such as a quarter wave plate, may be placed between the PBS 420 and the phosphor 428 and is reflected to increase the amount of light extracted by the PBS 420 as an effective output 434. The polarization of 440 is rotated.

  In an alternative configuration, the reflector may be arranged to reflect light transmitted by the PBS, while the light reflected by the PBS may be used as an effective output.

  Another exemplary embodiment of a lighting system 500 is schematically illustrated in FIG. 5A. This exemplary system is similar to system 400 illustrated in FIG. 4 except that PBS 420 and reflector 438 are replaced with a reflective polarizer layer 520, such as a MOF polarizer, a wire grid polarizer, or a cholesteric polarizer. ing. The reflective polarizer layer 520 may transmit light in one polarization state as an effective output 534 and reflect the light 536 in a state orthogonal thereto for reuse.

  Another exemplary embodiment of a lighting system 550 is schematically illustrated in FIG. 5B. This system 550 is similar to the system illustrated in FIG. 5A, except that the focusing optics 116 is eliminated and both the reflective filter 552 and the reflective polarizer 554 are curved. In some embodiments, the center of curvature of both the reflective filter 552 and the reflective polarizer 554 is near the phosphor 428, which may reflect light at both the first wavelength and the second wavelength. It may be desirable. This configuration increases the amount of first wavelength light 106 that is reflected back to the LED 102 and the amount of second wavelength light 536 reflected back to the phosphor 428. Of course, the center of curvature may be located elsewhere. The polarization rotation element 442 may be curved to match the curve of the reflective polarizer or may be straight. Similarly, curved reflective elements may be used in the other embodiments described above. For example, in the system 400 schematically illustrated in FIG. 4, the reflection filter 430 and the reflector 438 may each be curved.

  One feature of an illumination system that increases the amount of reflected back light for reuse is light of the first wavelength or light of the second wavelength. When the light reflected for reuse is reflected, The angle range of incident light is not substantially increased. This is further explained with reference to FIG. 5C, which shows the direction of the light rays at various points across the non-telecentric light beam propagating along axis 560. At the center of the beam, the central ray 562 is parallel to the axis 560 and the rays 564, 566 propagate at an angle α1 with respect to the central ray 562. Rays 564, 566 indicate a specific fraction of ray 562, in this case on-axis, of the maximum intensity of the ray. For example, if the light beam has an f / number of 2.4, the light beam is generally accepted as having a cone half angle of ± 11.7 ° (α1), which is at least 90% of virtually all. Excess light is contained within a ± 11.7 ° cone.

  Dashed line 570 at the beam edge is parallel to axis 560. Ray 572, which indicates the direction of the brightest ray at the beam edge, propagates at an angle θ relative to axis 560. Rays 574 and 576 propagate at an angle α2 with respect to ray 572. Ideally, the value of α2 is close to the value of α1, but they need not be exactly the same.

  Reflection of beam 562 by flat mirror 568 positioned perpendicular to axis 560 results in a reflected beam that propagates parallel to axis 560. On the other hand, the reflection of beam 572 by flat mirror 568 results in a reflected beam that propagates at an angle 2θ relative to axis 560. Therefore, reflection of non-telecentric light by a flat mirror results in an increase in the angular range of light.

  On the other hand, if the light is telecentric, then the beams 562 and 572 are parallel and reflection by the flat mirror 568 will not increase the angular range of the incident light.

  Also, as schematically illustrated in FIG. 5D, the non-telecentric reflection of light by the curved mirror 580 results in the case where the beams 562 and 572 are each normally incident on the mirror 580 and do not increase the angular range of the light. There is.

  One exemplary embodiment of a projection system 600 that may use an illumination source of the type described above is schematically illustrated in FIG. The system 600 illuminates each image forming device 604a, 604b, 604c, which includes a number of different colored light sources 602a, 602b, 602c, also referred to as image forming panels. Each light source 602a, 602b, 602c may include a number of light emitting elements, such as light emitting diodes (LEDs), to generate an outgoing light beam having a particular color. One or more light sources 602a, 602b, 602c are used to select a phosphor for converting the wavelength of light emitted by the LED, a concentrator for maintaining the etendue of the light beam, and a desired change state. A polarizer may be provided. In some embodiments, illumination light sources 602a, 602b, 602c generate red, green, and blue illumination light beams, respectively.

  The image forming apparatuses 604a, 604b, and 604c may be any suitable type of image forming apparatus. For example, the image forming apparatuses 604a, 604b, and 604c may be transmissive or reflective image forming apparatuses. Both transmissive and reflective liquid crystal display (LCD) panels may be used in the image forming apparatus. One example of a suitable type of transmissive LCD imaging panel is a high temperature polysilicon (HTPS) LCD device. One example of a suitable type of reflective LCD panel is a liquid crystal on silicon (LCoS) panel. The LCD panel modulates the illumination light beam by polarization modulating the light associated with the selected pixel and subsequently separating the modulated light from the unmodulated light using a polarizer. Another type of imaging device, called a digital multi-mirror device (DMD), is supplied by Texas Instruments, Inc., Plano, Texas, under the trade name DLP ™, which is an array of individually addressable mirrors. And the mirror reflects the illumination light in the direction of the projection lens or away from the projection lens. Illumination light sources may be used in both LCD and DLP ™ types of image forming devices, but are not intended to limit the scope of this disclosure to only these two types of image forming devices. The type of light source system described in may use other types of imaging devices projected by the projection system. It will also be appreciated that many systems, including DLP® type image forming devices, do not require polarized illumination light. The illustrated embodiment includes an LCD type image forming apparatus for illustrative purposes only and is not intended to limit the type of image projection system in which the illumination source is used.

  Illumination light sources 602a, 602b, 602c are mirrors for directing beam manipulation elements, eg, any of the colored illumination light beams 606a, 606b, 606c, to their corresponding image forming devices 604a, 604b, 604c. Or a prism may be included. The illumination light sources 602a, 602b, 602c may include various elements for dressing the illumination light beams 606a, 606b, 606c, such as polarizers, integrators, lenses, mirrors, and the like.

  The illumination light beams 606a, 606b, and 606c to be colored are directed to the respective image forming apparatuses 604a, 604b, and 604c via the respective polarization beam splitters (PBS) 610a, 610b, and 610c. Each of the reflected and colored image light beams 608a, 608b, and 608c is separated by PBS 610a, 610b, and 610c and moved to a color combiner unit 614 so that the image forming devices 604a, 604b, and 604c. The polarized light modulates the incident illumination light beam 606a, 606b, 606c. The colored image light beams 608a, 608b, and 608c may be combined into a single full color image beam 616 that is projected onto the screen 612 by the projection lens unit 611.

  In the exemplary embodiment illustrated, the colored illumination light beams 606a, 606b, 606c are reflected by the PBSs 610a, 610b, and 610c toward the image forming devices 604a, 604b, and 604c, and the resulting image light beams 608a, 608b and 608c are transmitted through PBS 610a, 610b and 610c. In another approach path not illustrated, the illumination light may be transmitted to the image forming apparatus via the PBS, while the image light is reflected by the PBS.

  One or more power supplies 620 may be connected to supply power to the illumination light sources 602a, 602b, 602c. In addition, the controller 622 may be coupled to the image forming devices 604a, 604b, 604 to control the image being projected. The controller 622 may be part of a stand-alone projector, or part of a television or computer, for example.

  In some embodiments, it may be desirable to use a densely packed array of LEDs, for example, to achieve light generation and collection efficiently and economically. Such an array may be arranged to have an aspect ratio similar to the image device being illuminated and to have an etendue at least as large as the image device being illuminated.

  One of the main challenges with high density LED packing in the array is the control of heat flux. To help control this heat load, the LED 702 may be directly attached to a liquid cooled plate 704 as schematically illustrated in FIG. The cooling plate 704 may be, for example, a liquid-cooled microchannel cooling plate having a liquid coolant inlet 706 and an outlet 708. The number of LEDs 702 mounted on the cooling plate 704 may be different from that shown in the figure. One suitable type of cooling plate 704 is a Normal flow microchannel cold plate (NCP) available from Mikros Technologies, Claremont, NH.

  An important feature of such an arrangement is that by attaching the LED 702 directly to the cooling plate 704, the thermal resistance to transfer the heat of the pn junction of the LED 702 to the liquid medium is reduced as much as possible. The LED 702 may be attached directly to the liquid cooling plate 704 using any suitable method, such as, for example, a flux eutectic die attach method or the use of a conductive epoxy.

  The eutectic flux method used to attach the LED chip 702 to the cooling plate 704 provides low electrical resistance, low thermal resistance, and good mechanical and electrical integrity. This is accomplished by placing a carefully adjusted amount of viscous flux on the cooling plate. Subsequently, the LED chip 702 is precisely arranged on the cooling plate via the viscous flux. The LED chip is provided with a metal coating on its lower surface. The metal coating may be, for example, an 80:20 Au / Sn mixture. The assembly 700 is heated beyond the melting point of the metal coating to reflow the metal and thereby attach the LED chip 702 to the cooling plate 704. In some embodiments, heating is performed for a short period of time, for example, reaching a temperature of about 305 ° C in 5-8 seconds.

  Traditionally, the reflow method is performed by directly heating the mounting substrate (eg, using a hot plate) or using a hot gas stream that is applied to the top of the chip. However, these conventional methods are not suitable for attaching the LED 702 to the cooling plate 704. It is difficult to heat the substrate to the reflow temperature and cool it again quickly enough to avoid excessive residence time at or near the processing temperature. If the chip is left at the reflow temperature for a long time, the Au / Sn may cause more flow than desired, and the metal may wick up the sides of the LED chip, causing unwanted short circuit or Schottky contact. contact). Furthermore, excessive residence time can cause complete or partial separation of the chip from the substrate, resulting in poor electrical and thermal contact.

  Conventional hot gas methods can be used to heat the chip and adjacent substrate in a finely controlled time, minimizing the possibility of short circuit formation or chip separation. However, this method is used for one chip at a time and requires 5-8 seconds of direct heating per chip. This method can be very time consuming for arrays containing many chips.

  Another approach for supplying heat in the reflow process is to control the flow of hot inert gas or hot liquid through the cooling plate. This method allows the entire plate 704 to be heated simultaneously, thereby allowing a batch process for greatly improved production throughput. Also, since the total thermal mass of the cooling plate 704 is not externally heated, it is easier to control the residence time at the reflow temperature, thus minimizing quality defects associated with excessive time at the reflow temperature.

This method allows the LEDs to be placed directly on the cooling plate, which is highly desirable. In the conventional method, the LED is mounted on the intermediate substrate, and then it is mounted on the heat sink. This results in an extra layer of intermediate substrate material and additional thermal resistance due to extra thermal interfaces. At high flux density, this extra resistance can substantially increase the pn junction temperature (T _j ) of the LEDs 702 in the array.

By removing the intermediate substrate and the thermal interface, the thermal resistance between the diode junction and the coolant is reduced, so that the junction operates at a lower temperature. This reduction in operating temperature provides at least two advantages. First, because the lifetime is related to T _j , the lifetime of the LED also increases while operating at high power. Therefore, keeping the LED chip cooler increases reliability. Second, a higher value of T _j adversely affects the amount of light emitted by the LED. By keeping T _j lower, the brightness from the LED array is higher for a given incident power.

  One example of an LED array mounted directly on the cooling plate is 84 LEDs arranged in a 12 × 7 array. Each LED is a type 460XT290 blue LED and is supplied by Cree Inc. of Durham, NC. Each LED is 300 square micrometers, and these are mounted with a center-to-center spacing of 325 micrometers. Thus, the array is approximately 3.9 mm by 2.25 mm. The LED chip is relatively thin and is about 110 μm high, although higher LEDs can also be used. For example, a Cree Type XB900 LED chip (900 square μm × 250 μm high) may be used. A wire bond wire with a diameter of 25-50 μm is attached to the top of the LED to provide electrical connection. The wire may be formed of any suitable material such as gold. The cooling plate serves as a common ground for all LEDs.

  The phosphor may be conformally coated on the LED. The phosphor material may be coated using any suitable method. Some suitable “wet” methods include spraying phosphor material onto the LED and dipping in a slurry. Other methods of applying phosphors, such as vacuum coating, electrophoresis, or KaSil settling, may be used.

Because phosphor wavelength conversion is often low efficiency at higher temperatures, it is important to keep the temperature of the phosphor as low as T _j . Configurations in which the phosphor is conformally coated on the LED can enhance phosphor cooling: the special Cree LED discussed above is made of silicon carbide, which is a relatively high thermal conductivity. Thereby reducing the thermal resistance of the thermal path between the phosphor and the cooling plate. Thus, another advantage of reducing the thermal resistance between the LED and the liquid coolant is that the phosphor can be thermally connected to the cooling system via the LED.

  The array is then placed as close as possible to the tapered light pipe. If the wire bonding is a “wedge” type rather than a “ball” type, the height of the wire bonding is reduced, so that the entrance surface of the tapered light pipe is as close as about 100 μm from the top surface of the LED chip. You may arrange. The incident end of the tapered light pipe may have an incident dimension of about 2.25 mm × about 3.9 mm. The length of the tapered light pipe may be in the range of 50 mm to 60 mm, but other lengths may be used. In the example where the exit surface has a surface that is 1.8 times larger than the entrance surface, the exit surface has a dimension of about 7.05 mm × about 4.1 mm.

  Another embodiment of the illumination light source 800 is schematically illustrated in FIG. The illumination light source 800 has some similarities to those illustrated in FIG. 5B. However, in this embodiment, some blue light 856 is emitted from the illumination source 800 along with the green light 534. Accordingly, the reflective filter 852 is optional: it may be removed entirely or provided to pass some fraction of the incident blue light 106. Further, the reflective polarizer 854 may be effective for both green and blue light wavelengths.

  The amount of active species in the phosphor may be adjusted, for example, by adjusting the thickness of the phosphor and / or by adjusting the concentration of the active species in the phosphor. Thus, by adjusting the phosphor, the phosphor may generate a desired amount of light at the second wavelength, while still having a configuration that transmits a specific amount of light at the first wavelength. In this manner, the illumination light source 800 may generate an output that includes light of both the first and second wavelengths. Under certain conditions, the ratio of the luminance of the first wavelength light to the second wavelength light may be selected to achieve a particular color temperature. For example, ITU-R BT., Published by the International telecommunications Union (ITU). According to the 709 standard, a neutral white point with no color bias is achieved when the luminance of incident light on the screen includes 7% blue light, 70% green light and 23% red light. Therefore, an illumination light source of the type that generates an outgoing light beam including the green light and the blue light illustrated in FIG. 8 at a ratio of 10: 1 is ITU-R BT. It can be used as a blue / green light source in systems conforming to the 709 standard. According to some other color standards having higher color temperatures, the amount of blue light constitutes 12% of the amount of green light.

  By controlling the phosphor to transmit about 10% to about 15% of the incident blue light, the light emission from the illumination light source is unbiased and irradiates the green and blue image forming apparatuses in the three-color projection system It becomes suitable for doing.

  In addition to the embodiment illustrated in FIG. 8, other embodiments of the illumination light source described above generate an output that includes light of both the first and second wavelengths, for example, by removing or replacing a color filter. It will be understood that it can be adapted to.

  Illumination light sources that produce two colors of light may be used in many different types of projection systems. For example, a dual color illumination light source 800 with suitable color separation elements can be used to replace the two illumination light sources 620a, 602b, or 602c in the projection system 600. A dichroic separator may be used to separate the dual color outgoing beam into light beams of different colors.

  Another exemplary projection system 900 in which such an illumination source may be used is schematically illustrated in FIG. Light 904 from the illumination source 902 that produces an outgoing beam that wraps both blue and green light is split by the dichroic beam splitter 906 into respective green and blue beams 908, 910. These beams are directed to respective transmissive image forming apparatuses 912, 914, for example, transmissive LCD units. The transmissive image forming apparatuses 912 and 914 may include a polarizer to remove non-image light.

  Another illumination light source 916 generates red light 918 that is directed to the transmissive image forming device 920. Image light from the three image forming devices is combined together in a color combiner unit 922, such as an X-cube combiner, or other suitable combiner, and projected onto a screen by a projection lens system 926. A beam 924 is generated.

  An illumination light source 1000 that generates these different colors is schematically illustrated in FIG. The illumination light source 1000 is similar to that illustrated in FIG. 8, with the exception that the LED array comprises two types of LEDs 102a, 102b, each producing a respective colored light. For example, LED 102a may generate blue light while 102b may generate red light. Therefore, the LEDs 102a and 102b respectively generate the first wavelength light 106 and the third wavelength light 1002 indicated by the solid line and the dotted line, and at least a part of the first wavelength light by the phosphor 428 as indicated by the broken line. Is converted into light 432 of the second wavelength. In this case, the reflective polarizer 1054 is effective over all three wavelengths, substantially transmitting three different wavelengths of light in one polarization state, and three different in orthogonal polarization states. Reflects light at wavelengths. Accordingly, the reflective polarizer 1054 reflects a portion 1004 of the second wavelength light and transmits another portion 1006 of the second wavelength light.

  Typically, if the loss of second wavelength light 1002 as it passes through phosphor 428 is low due to, for example, a low or negligible absorption coefficient, a high fraction of third wavelength light 1002 is obtained. Passes through phosphor 428. Thus, the phosphor 428 may be formed over the entire array of LEDs 102, 102b, even if the phosphor is effective in wavelength converting only the light 106 generated by the first LED 102a. In other embodiments, the phosphor 428 may cover only the LED 102a having the converted wavelength of light, for example, without covering the LED 102b that does not convert the light.

  In some embodiments, the first LED 102a generates blue light, a portion of which is converted to green light by the phosphor, and the second LED 102b generates red light. Accordingly, the illumination light source 1000 can generate light of three different wavelengths. The color balance of the light generated by the three-color illumination source 1000 can be achieved, for example, by adjusting the number of red LEDs 102b relative to the number of blue LEDs 102b and / or by adjusting the power applied to the two sets of LEDs 102a and 102b. You can choose. It may be desirable to set the white point of the three color light emitted by the illumination light source 1000 to a desired white point, such as that set by a professional standard. For example, the Society of Movie Picture and Television Engineers (SMPTE) has recommended standards 145-1999, “C-Color Monitor Color Measurements (C-TE)” which describe recommended color gamuts for television and movie images. Color Monitor Colorimetry) ”. According to this criterion, neutral white has color coordinates near (300, 310) using the CIE 1931 color coordinate system.

  A three-color illumination source may be used for many applications, including illuminating a single-plate projection system. In such a system there is one image forming device used to continuously generate red, green and blue images. These images are integrated to form a full-color image on the observer's eye. The single-plate projection system may be based on a liquid crystal image forming device or other type of image forming device. For example, a three-color illumination source may be used in a single plate projection system.

  Therefore, the present invention should not be considered limited to the specific examples given above, but rather should be understood to cover all aspects of the invention as appropriately specified in the appended claims. It is. Upon review of this specification, various modifications, equivalent methods, and numerous structures to which the present invention may be applied will be readily apparent to those skilled in the art to which the present invention is directed. The claims are intended to cover such modifications and devices.

  The present invention may be more fully understood in view of the following detailed description of various embodiments of the invention in conjunction with the accompanying drawings.

  While the invention is susceptible to various modifications and alternative forms, specifics thereof are shown by way of example in the drawings and will be described in detail. It will be understood, however, that the intention of the invention is not intended to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

Fig. 3 schematically illustrates an exemplary embodiment of an illumination light source according to the principles of the present invention. Fig. 3 schematically illustrates an embodiment of an array of light emitting diodes as part of an illumination light source. Fig. 5 schematically illustrates another exemplary embodiment of an illumination light source according to the principles of the present invention. Fig. 5 schematically illustrates another exemplary embodiment of an illumination light source according to the principles of the present invention. Fig. 4 schematically illustrates an additional exemplary embodiment of an illumination light source according to the principles of the present invention. Fig. 4 schematically illustrates an additional exemplary embodiment of an illumination light source according to the principles of the present invention. Illustrates the reflection of light by another type of reflector from which light diverges. Illustrates the reflection of light by another type of reflector from which light diverges. 1 schematically illustrates an embodiment of an image projection system using an illumination light source according to the principles of the present invention. Fig. 3 schematically illustrates an embodiment of an array of LEDs on a cooling plate according to the principles of the present invention. Fig. 4 schematically illustrates another illumination source that produces an output including light of two wavelengths according to the principles of the present invention. 9 schematically illustrates an exemplary embodiment of a projection system using an illumination light source of the type illustrated in FIG. Fig. 5 schematically illustrates another illumination source embodiment that produces an output comprising three wavelengths of light according to the principles of the present invention.

Claims (27)

An arrangement of one or more LEDs including a light emitting diode (LED) capable of generating light of a first wavelength;
A phosphor material disposed proximate to the one or more LEDs and emitting light of a second wavelength when illuminated by light of a first wavelength;
A condensing unit comprising at least a tapered optical element, through which light of the first wavelength and the second wavelength passes;
It is arranged to transmit light of the first wavelength and the second wavelength in the first polarization state and reflect light of the first wavelength and the second wavelength in the second polarization state orthogonal to the first polarization state. A reflective polarizer,
The first and second wavelengths of light reflected by the reflective polarizer are directed to the phosphor material without substantially increasing the angular range;
An illumination light source that generates an outgoing light beam including polarized light having a first wavelength and a second wavelength.   The illumination light source of claim 1, wherein the first and second wavelengths of light are substantially telecentric in the reflective polarizer.   The illumination light source of claim 1, wherein the reflective polarizer comprises a curved reflective polarizer.   The illumination light source according to claim 3, wherein the curved reflective polarizer has a radius of curvature centered in the vicinity of the phosphor material.   The illumination light source according to claim 1, wherein the first wavelength is a blue wavelength and the second wavelength is a green wavelength. The illumination light source according to claim 1, wherein the phosphor material includes Eu-doped SrGa ₂ S ₄ .   The illumination light source of claim 1, wherein the tapered optical element comprises a flat reflective side.   The illumination light source of claim 1, wherein the tapered optical element comprises a curved reflective side.   The illumination light source according to claim 1, wherein the condensing unit includes a lens disposed in the vicinity of the exit end of the tapered optical element.   The illumination light source of claim 1, wherein the tapered optical element comprises a transparent body having an internal reflective sidewall and a curved exit surface.   And further comprising a reflective element arranged to reflect a first portion of the first wavelength of light that has passed through the phosphor material and to transmit a second portion of the first wavelength of light that has passed through the phosphor material. Item 2. The illumination light source according to Item 1.   The illumination light source of claim 1, wherein the reflective polarizer comprises a multilayer optical film polarizing beam splitter and a mirror.   The illumination light source of claim 1, wherein the reflective polarizer comprises one of a wire grid polarizer and a cholesteric polarizer.   The illumination light source according to claim 1, further comprising a polarization control element disposed between the phosphor and the reflective polarizer.   The illumination light source of claim 1, wherein the reflective polarizer comprises a multilayer optical film polarizer. The one or more LEDs are arranged at an interval from the incident side to the light collecting unit,
The reflector further comprises at least partially surrounding the incident side so as to reflect light from the one or more LEDs towards the incident side that would not enter the incident side without the reflector. The illumination light source described in 1.   The illumination light source of claim 1, further comprising a power source connected to supply power to the one or more LEDs.   The illumination light source according to claim 1, wherein the arrangement of the one or more LEDs further includes an LED capable of generating light of a third wavelength.   The reflective polarizer transmits light of the first, second, and third wavelengths in the first polarization state, and reflects light of the first, second, and third wavelengths in the second polarization state. The illumination light source according to claim 18, wherein the outgoing beam further includes polarized light of a third wavelength.   The illumination light source according to claim 1, further comprising an image forming apparatus, wherein the second wavelength light is directed toward the image forming apparatus.   The illumination light source according to claim 20, further comprising: a projection lens unit arranged to project an image from the image forming apparatus; and a screen on which the image is projected.   The illumination light source of claim 20, further comprising a controller connected to control an image formed by the image forming apparatus. An array of one or more LEDs including a first light emitting diode (LED) capable of emitting light of a first wavelength and a second LED capable of emitting light of a second wavelength different from the first wavelength;
A phosphor material disposed on the first LED such that light from the first LED is incident on the phosphor material;
An illumination light source in which light having a first wavelength is converted into light having a third wavelength by a phosphor.   24. The illumination light source of claim 23, further comprising a condensing unit arranged to receive light of the first, second and third wavelengths. Further comprising a liquid cooled plate having a first surface and a second surface;
24. The illumination light source of claim 23, wherein the first and second LEDs are directly attached to the first surface of the liquid cooling plate and a liquid coolant is in contact with the second surface of the liquid cooling plate.   The illumination light source of claim 25, wherein the liquid cooling plate comprises a metal plate having a thickness of less than 1 mm.   The illumination light source according to claim 23, wherein the light of the second wavelength passes through the phosphor.

Images